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Why preparation matters more than the paint itself

Ask any decorator with a few decades behind them and they will tell you the same thing: the finish you get on exterior woodwork is decided long before the primer tin is opened. Primer is a bonding layer, and it can only bond to what it is given. Apply it over flaking paint, dusty timber or a greasy window frame and you have wasted both the primer and your Saturday. Get the preparation right, though, and the primer will grip properly, seal the grain, and give the topcoats a fighting chance against everything a British winter can throw at them.

Exterior joinery takes a beating. Sun, rain, frost, alternating wet and dry cycles — wood moves constantly, and any weakness in the coating system will show up within a season or two. The good news is that preparation is mostly straightforward physical work. No special skills, just patience and the right approach.

Start with a proper survey of the woodwork

Before you touch a scraper, walk around the property with a screwdriver and a critical eye. Press the tip into any suspect timber. Sound wood resists; soft, spongy wood gives way, which usually means rot has taken hold beneath the surface. Rot needs cutting out and replacing before any coating goes anywhere near it — primer over decay is simply hiding a problem that will grow.

Check for other issues while you are there:

• Movement and gaps — failed mastic around frames, or joints that have opened up.
• Damp staining — dark patches or a greenish tinge suggest water is getting in from above.
• Loose glazing putty — crumbling putty around panes lets water sit against the rebate.
• Rusty fixings — old nails and screws bleed through paint and need spot priming.

Deal with these first. There is no point spending hours sanding a sill if water is still running down the wall behind it.

Scraping back loose and flaking paint

The aim of scraping is not to strip every surface back to bare timber. It is to remove anything that is not firmly attached, and to feather the edges of what remains so you cannot feel a ridge when you run your fingers across it.

Use a stiff-bladed shaker scraper for flat surfaces and a shaped or pull scraper for mouldings, beads and curved sections. Work with the grain where you can, and let the blade do the work rather than forcing it. On painted timber that has blistered or cracked, a hot air gun on a low setting softens the coating and makes removal far quicker, but keep the nozzle moving — scorching the wood underneath is easy and it will affect how the primer takes.

Anywhere paint has failed, ask why. Paint rarely flakes for no reason. Usually it is moisture getting in from behind, a previous coat applied over a damp or dirty surface, or simply decades of build-up becoming brittle. Once you have scraped back to a sound edge, feather it with a medium abrasive — around 80 to 120 grit — so the transition from old paint to bare wood is smooth rather than stepped.

Sanding bare timber the right way

Bare timber needs abrading, but not over-sanding. The goal is a clean, slightly keyed surface with the grain closed and no raised fibres, because primer soaks into open grain unevenly and can leave a patchy base for the topcoats.

• Start with 80 grit on rough or weathered timber to remove grey, weathered fibres and flatten minor dents.
• Move to 120 grit to refine the surface and remove the scratches left by the coarser paper.
• Finish with 150 grit on softwoods, which are easily marked by anything heavier.

Always sand along the grain. Cross-grain scratches are almost impossible to hide once paint is on, because the primer and topcoats sit in the grooves and catch the light differently. On mouldings and tight profiles, wrap the paper around a shaped block or use a foam pad so you do not round over crisp edges. Sharp arrises hold paint well; over-sanded, rounded ones wear through quickly.

When you have finished, dust off thoroughly. Brush down, vacuum if you can, then wipe with a cloth dampened with white spirit on resinous timber. Primer will not stick to dust — it is that simple.

Treating knots and resinous patches

Knots are the single most common cause of a disappointing exterior paint job. Resin in softwood knots migrates through coatings, and because it is oily it will lift primer and topcoats from underneath, leaving brown stains and eventually blisters and flaking.

On new or bare timber, seal every knot and any resinous streak with a dedicated knotting solution, shellac-based sealer or a stain-blocking primer. Apply two thin coats rather than one thick one and let it dry fully before overcoating. Do not assume a general-purpose primer will hold knots back — many will not, and you will only find out months later when the brown patches appear.

If a knot is weeping on an older piece of joinery, scrape away the soft resin first with a scraper or a rag soaked in white spirit, allow the area to dry, then seal. Resinous timber such as pitch pine needs a sealer across the whole surface, not just over the knots.

Priming while the wood is dry and the weather is kind

Preparation is worthless if you prime at the wrong moment. Timber needs to be genuinely dry — not dry on the surface after a shower, but dry through. A moisture meter takes the guesswork out of it, and a reading below around 16 to 18 per cent is a sensible target for exterior joinery.

Check the forecast and give yourself a decent window. Most solvent-borne and water-based exterior primers want temperatures above 8 to 10°C, and they need time to cure before rain arrives — often several hours, sometimes overnight, depending on the product. Prime bare patches and filled areas the same day you sand them where possible, so the timber does not pick up moisture or dirt in between. And apply the primer thinly on the first coat, working it into the grain, rather than laying it on thick and hoping for the best. Thin coats grip; thick ones sit on the surface and fail.

Do all of that, and the rest of the system — undercoat and gloss, or whatever finish you have chosen — will be sitting on a foundation that can genuinely last.